Microscale fluid systems are widespread in bioanalysis, and also appear in energy systems such as fuel cells. Reynolds numbers in such systems are very low, so molecular mixing enhancement cannot exploit turbulent effects. Instead, it is necessary to apply concepts of chaotic advection, which are familiar in macroscale laminar flows. This SGER will address passive, steady-flow micromixers, though the general approach will also be applicable to active mixers that generate unsteady flows through, for example, mechanical actuation. There are numerous micromixer designs that aim to generate chaotic advection, which are often adaptations of two-dimensional (2-D) chaotic flow geometries to three-dimensional (3-D) microchannels. Conceptually, flow trajectories are compared by treating successive transverse sections of 3-D channels equivalently to temporal series in 2-D flows Existing efforts to quantify microchannel flows rely on single-point or field measurements of flow velocities, using techniques such as micro-PIV. Such measurements are fundamentally mismatched with flow configurations based on 2-D chaotic flow analogies, however, because many tools for analyzing chaotic flows rely on knowledge of particle trajectories (or streaklines), and of their mappings into lower-dimensional space. Determining trajectories or streaklines from velocity measurements requires knowledge of the temporal evolution of the 3-D velocity field, which remains challenging for micro-PIV. The PI will demonstrate a new method for quantifying flow fields in channel-type micromixing devices, where the method employs single-molecule detection techniques to measure flow streaklines directly. Streakline measurements inherently offer more direct insight into mixing than velocity field measurements, by allowing access to established tools of chaotic flow analysis. For example, The PI will be able to construct Poincare sections explicitly to assess the chaotic properties of microchannel flows by considering the divergence of initially proximal streaklines, or the properties of fixed points, etc. These measurements will eventually provide a rigorous, yet intuitive framework for the design and characterization of micromixing devices.
